1. Technical Field
The present invention relates to a Coriolis flow meter which detects a phase difference and/or a vibration frequency proportional to a Coriolis force applied to a curved flow tube to thereby obtain a mass flow rate and/or the density of a fluid flowing through the flow tube, and in particular a Coriolis flow meter which is suitable for minute flow rate measurement.
2. Background Art
The Coriolis flowmeter is a mass flowmeter which utilizes the fact that when a flow tube through which a fluid flows is supported at one end or both ends thereof, and vibration is applied to a portion of the flow tube around the supporting point of the flow tube in a direction perpendicular to the flow direction, the Coriolis forces applied to the flow tube are proportional to the mass flow rate. Coriolis flowmeters, which are well known, are roughly classified in two types according to flow tube structure: a straight-tube type, and a curved-tube type.
In a Coriolis flowmeter of the straight-tube type, when vibration is applied to a straight tube, of which both ends are supported, in a direction vertical to the straight-tube center portion axis, a difference in displacement due to the Coriolis forces is generated between the support portions and the central portion of the straight tube, that is, a phase difference signal is obtained. Based on this phase difference signal, the mass flow rate is detected. The straight tube type Coriolis flowmeter thus constructed has a simple, compact, and solid structure. On the other hand, there arises a problem in that it is difficult to achieve high detection sensitivity.
In contrast, the curved tube type Coriolis flowmeter is superior to the straight tube type Coriolis flowmeter from a view point that the curved tube type Coriolis flowmeter allows selection of a shape for effectively obtaining the Coriolis forces. In fact, the curved tube type Coriolis flowmeter is capable of performing mass flow rate detection with high sensitivity. Known examples of the curved tube type Coriolis flowmeter include one equipped with a single flow tube (see, for example, JP 04-55250 A), one equipped with two flow tubes arranged in parallel (see, for example, JP 2939242 B), and one equipped with a single flow tube in a looped state (see, for example, JP 05-69453 A).